Fix run-editor bug with cmd2 >= 2.0.0 compatibility

In cmd2, the upstream authors decided to rename a method in 2.0.0
without providing a backwards compatibility wrapper.  Let's add that
locally.

Change-Id: Iaa17b93db13ba330551799cce5f0388c78217224
Closes: OS#6071
diff --git a/pySim/filesystem.py b/pySim/filesystem.py
index b5d0ccd..04e849b 100644
--- a/pySim/filesystem.py
+++ b/pySim/filesystem.py
@@ -649,7 +649,7 @@
                 with open(filename, 'w') as text_file:
                     json.dump(orig_json, text_file, indent=4)
                 # run a text editor
-                self._cmd._run_editor(filename)
+                self._cmd.run_editor(filename)
                 with open(filename, 'r') as text_file:
                     edited_json = json.load(text_file)
                 if edited_json == orig_json:
@@ -917,7 +917,7 @@
                 with open(filename, 'w') as text_file:
                     json.dump(orig_json, text_file, indent=4)
                 # run a text editor
-                self._cmd._run_editor(filename)
+                self._cmd.run_editor(filename)
                 with open(filename, 'r') as text_file:
                     edited_json = json.load(text_file)
                 if edited_json == orig_json: